CD40 is a cell surface receptor, which belongs to the TNF-R family, and which was first identified and functionally characterized on B lymphocytes. However, in recent years it has become clear that CD40 is more widely expressed, including expression on monocytes, dendritic cells, endothelial cells and epithelial cells. Therefore it is now thought that CD40 plays a more general role in immune regulation.
The CD40 protein is a 45–50 kDa glycoprotein of 277 AA, the 193 AA extracellular domain is composed of four imperfect repeats of about 40 residues, anchored by a superimposable pattern of six cysteines. This organization is found in the other members of the TNF-R family.
CD40 is expressed by multiple cell types. In the hematopoietic system, it is expressed on CD45 hematopoietic progenitors, B cell progenitors, mature B lymphocytes, plasma cells, monocytes, dendritic cells, cosinophils, basophils and on some T lymphocytes. CD40 is also expressed on non-hematopoietic cells such as endothelial cells, fibroblasts and epithelial cells.
Expression cloning using a CD40-Fc fusion protein allowed the isolation of a CD40-ligand (CD40-L) from activated T cells. The human CD40-L is a polypeptide of 261 AA including a 215 AA extracellular domain with five cysteins. CD40-L is a member of the Tumor Necrosis Factor family.
CD40-R bound to its ligand activates protein-tyrosine kinases, including Lyn and Syk and induces the tyrosine phosphorylation of multiple substrates including phosphatidylinositol 3-kinase and phospholipase Cγ2.
CD40 also interacts with TRAF3 protein (TNF-R Associated Factor-3), also identified under the name CRAF1, CD40 bp, LAP1 and CAP1 (18-21). TRAF3 is a 62 kD intracellular protein which is expressed in almost all cell types. The protein contains several functional domains which might be involved in signal transduction.
In addition, CD40 has been demonstrated to associate with TRAF2, a molecule which also associates with TNF-R2. The induction of NF-κB activation via CD40 cross-linking (and also via TNF-R2) could be attributed to TRAF2 signaling.
CD40 ligation activates resting B cells as shown by increase in size and expression of new surface molecules involved in homotypic and heterotypic aggregation (CD23, VLA-4), as well as T cell costimulation (CD80/CD86). Furthermore, CD40-activated B cells secrete a panel of cytokines which may act as autocrine and paracrine growth and differentiation factors.
CD40 is further known to be expressed on professional antigen presenting cells such as monocytes and dendritic cells, and its ligation to the above cells results in secretion of multiple proteins including cytokines such as IL1, IL5, IL8 IL10, IL12, TNFα, MiPlα, as well as enzymes such as matrix metalloproteinase (MMP).
The importance of this receptor-ligand pair for the cellular immune response, has been demonstrated by the diminished immunity against several pathogens in CD40 and CD40-L knockout mice. In keeping with this, CD40 ligation turns on monocyte tumoricidal activity as well as NO synthesis.
Administration of antibodies to CD40-L has been shown to prevent the establishment of autoimmune symptoms in various murine models including: (1) collagen type II-induced arthritis: a model for human rheumatoid arthritis; (2) lupus nephritis in lupus prone mice that represent models for the systemic lupus erythematosus; (3) protcolipoprotein induced experimental encephalomyelitis: a model of human multiple sclerosis.
Administration of anti-CD40-L antibodies further has been demonstrated to prevent the development of graft versus host disease (GVHD) that occurs as a major complication of allogeneic bone marrow transplantation (van Kooten, C. and Banchereau, J., Frontiers in Bioscience, d1-11 Jan. 1, (1997)).